Rain Water Harvesting

Rain: a valuable resource and a problem

The Northwest gets a lot of rain in the winter. We get so much that it sometimes causes problems like flooding, sewer overflows, stream erosion, and carrying urban pollution into our waterways. But in the summer we get very little rain (less than Tucson, Arizona). So, it makes sense to conserve.

Follow these links to learn how to reduce runoff and protect our waterways:

Rain water harvesting usually involves larger cisterns or multiple barrel systems that can store enough water to help water landscapes during our long dry summers. Simple practices like amending soil with compost, mulching, and smart watering are the first steps to storing and conserving water.

Using rain water is easy — and Seattle residents can get a great deal on rain barrels.

How much rain water can I catch?

The Puget Sound averages about three feet of rain per year, but two thirds of it falls from November to March. Most areas in the region average less than two inches total rainfall for July and August.

To determine the amount of rain your roof catches, multiply your home’s width by its length (in feet) to estimate its footprint. Then estimate the portion of this area that drains to the downspout you’ll be using to catch your rain.

This formula will give a rough estimate of how much rain you can catch:

Rain caught (gallons) = (inches of rain) x 0.6* x (portion of building footprint).*One inch of rain falling on a square foot of surface yields approximately 0.6 gallons of water.

For example, if your home’s footprint is 1,400 square feet, and you want to know the amount of water that comes from a ¼ inch (.25”) rain event:

Rain caught (gallons) = (.25) x (.6) x (1,400) = 210 gallons(or less if you’re only gathering from one part of the roof).

Storage, however, is limited to the capacity of your system. Added capacity helps your system weather dry spells, although most homeowners don’t have room to store the thousands of gallons they use in landscape irrigation through our dry summers. And, the large cisterns to do it would take a very long time to pay back. Capacity and cost are directly related: decide how much you want to spend on storage. Natural Yard Care practices like building soil with compost and mulching, choosing low-water use plants, and Smart Watering practices all have much shorter paybacks, and grow healthier lawns and gardens too. So use all those practices and simple indoor conservation practices (see the Saving Water Partnership) first before investing in big rainwater collection systems.

You can help solve our winter stormwater runoff problems by connecting a hose to the drain valve on your rain barrel or cistern and running it out into a lawn or landscape. Then just open the drain valve in October, and let the barrel slowly drain out between big storms to slow our runoff all winter. In May, close the drain valve and let your barrel fill up to store water for landscape watering during our dry summers. Learn more about how you can help at RainWise.

Learn more about rain water harvesting systems by following the links below, or search the Web under “rain water harvesting.”